1. Field Of The Invention
This invention relates, in general, to amplifiers and, more specifically, to audio power amplifiers.
2. Description Of The Prior Art
Audio power amplifiers generally employ one of two different types of amplification devices, i.e., vacuum tubes or transistors. Both types have their advantages and disadvantages.
In a vacuum tube amplifier, superior audio response is attainable, however, at the expense of large size, weight and power consumption. Conventional bipolar or junction transistor based amplifiers while providing a significant reduction in the size and weight of the amplifier, have not been able to match the audio characteristics of vacuum tubes in providing a wide range of audio reproduction and amplification.
One of the primary factors in a vacuum tube amplifier's ability to accurately reproduce audio frequencies over a wide range is its ability to be connected in a transconductance mode of operation in which the output current through the output stage of the vacuum tube amplifier is a direct function of the input voltage to the amplifier. Such a circuit configuration isolates the output current from the input drive voltage and provides voltage amplification between the output and input voltages and simplicity in the construction of the input drive circuit. Such an amplifier circuit configuration is not possible utilizing bipolar type transistors.
A more recent development in the field of solid state circuitry, the field effect transistor (FET) and, more particularly, the power MOS-FET, have also been employed in audio amplifiers since they possess essentially the same operating characteristics as vacuum tubes. However, field effect transistors have, up to now, been employed in audio power amplifiers in the same circuit configurations as bipolar transistors, i.e., in an emitterfollower output connection, which effectively causes such FET-based amplifier output stages to behave as bipolar transistors with all of their limitations.
Thus, it would be desirable to provide an audio power amplifier having a transistor based output stage which matches the audio reproduction cabilities previously attainable only with vacuum tubes. It would also be desirable to provide an audio power amplifier in which power output transistors can be connected in the same output stage arrangement as vacuum tubes.